fix(api): fix 'winborder' preventing splits with nvim_open_win (#32981)

While at it, rename the p_winbd variable to p_winborder, as 'winbd' is
not the option's short name.

Co-authored-by: glepnir <glephunter@gmail.com>

src/nvim/options.lua

@@ -10205,7 +10205,7 @@ local options = {
       scope = { 'global' },
       short_desc = N_('border of floating window'),
       type = 'string',
-      varname = 'p_winbd',
+      varname = 'p_winborder',
     },
     {
       abbreviation = 'wi',